In a communication system, before a signal is modulated onto a carrier wave having an intermediate frequency, it is referred to as a baseband signal. In a transmitter, the baseband signal may be split into I and Q components that make up a vector defining the information signal. The I and Q components are modulated onto a carrier wave using a modulator and the output is up-converted using one or more frequency mixers. The carrier wave includes the amplitude and phase components of the modulating signal. Because the modulator operates at relatively low power levels compared to the transmitted power level, amplification between the modulator and the antenna is necessary. This amplification should be linear and efficient. Non-linear amplification creates distortion that may cause, among other things, error in the information vector. In a worst case, distortion may cause broadening of the frequency spectrum of the transmitted signal. Broadening of the frequency spectrum may interfere with nearby channels and may reduce traffic capacity. It may also result in the consumption of additional power reducing the efficiency of the transmitter which is undesirable, especially for mobile communication devices.
Linear amplifiers have been used to help improve the linearity of the output signals, but their efficiency is too low to be a practical alternative to non-linear power amplifiers, especially for mobile communication devices. Pre-distortion of the I and Q components is another technique that has been used to improve linearity but it is difficult to implement and it's application is limited. Cartesian feedback is another technique used for improving linearity, however this technique requires exact phase matching at the power amplifier output.
Polar-loop transmitters have also been used to help reduce spectrum broadening and improve power amplifier linearity. In a conventional polar-loop transmitter, an information signal is split into its polar components which consist of a phase reference component and an amplitude reference component. The components are processed in separate control loops and recombined to produce an output signal. One problem with conventional polar-loop transmitters is that modem communication techniques introduce modulation schemes, including for example, Code Division Multiple Access (CDMA) and Wideband (CDMA) schemes, where the instant signal trajectory may cross the zero point on a phasor diagram. This zero-crossing trajectory creates several difficulties for conventional polar-loop transmitters. For example, a zero-crossing trajectory has a phase component discontinuity similar to a step-function that results from the instantaneous transition of the phase by 180 degrees. The amplitude component at this zero-crossing occurrence may also contain a time derivative discontinuity. Because of the zero crossing, both the amplitude and phase components become very wideband making the processing of these components sensitive to bandwidth and slew-rate limitations.
Another problem with conventional polar-loop transmitters is the processing of the phase component by a phase detector. A phase detector's transfer function typically depends on the amplitude of the incoming signal, and amplitude regulating circuitry removes the amplitude modulation component. The amplitude regulating circuitry, such as a limiter or an automatic level control (ALC) circuit, has a limited range of functionality and has difficult handling zero-crossing trajectories.
Thus there is a general need for an improved transmitter and method for transmission of signals. There is also a need for a transmitter and method for transmission of signals with improved efficiency. There is also a need for a transmitter and method for transmission of signals that helps reduce the broadening of the frequency spectrum. There is also a need for a transmitter and method for transmission of signals that helps increase the efficiency of a transmitter's non-linear amplifier. There is also a need for a transmitter and method for transmission of signals with improved linearity. There is also a need for an improved polar-loop transmitter and method that handles zero-crossing trajectories.